Automotive anti-skid control system



Feb. 10, 1970 M. sLAvlN ET AL AUTOMOTIVE ANTI-SKID CONTROL SYSTEM Filed March 15, 1968 6 Sheets-Sheet 1 BY 'ZLW a A ORNEY Feb. 10,1970 M SLAVI'N mL 3,494,671

AUTOMOTIVE NTI-SKID CONTROL SYSTEM Filed March 13, 1968 6 Sheets-Sheet 2 E/G. 3. I H612.,

//5 SL/P 0 VEHICLE MPH`P 5L /P` VEH/C/ E VELOCITY f WHEEL vELoc/ry Y II a In LL A E c DEF G H J K FIG' 5' INVENTORS RALPH W CARP M/OHEL SLAV//V Feb.'10, 1970 M. sLAvlN ET AL AUTOMOTIVE ANTI-SKID CONTROL SYSTEM 6 Sheets-Sheet 3 Filed March 13, 1968 NSM INVENTORS RALPH W CARP M/OHEL SLV//V ,i ORNEY vQm.

M. sLAvlN ET AL AUTOMOTIVE ANTI-SKID CONTROL SYSTEM Filed'MaIOh l5, 1968 6 Sheets-Sheet 4 RALPH I4. CARP M/OHEL SLV//V INVENTORS Feb..l0, 1970l Filed March 13, 1968 .om NNN |I1 or m. Nn Y .mw @h Gbm mkm# m N m m P W R @NN M .DAN A n C mbv A W EN M M M m QN R M N M m Y N\N V B Nm \v\ .QN QN m msm v L F M,

Feb. l0, 1970 I M. sl- Avm T AL AUTOMOTIVE ANTI-SKID CONTROL SYSTEM 6 Sheets-Sheet 6 F'led March l5, 1968 INVENTORS RALPH W CARP M//'ELl SLV//V ZW QJ/@M TTORNEY BYZ United States Patent O 3 494,671 AUTOMOTIVE ANTI-SKID CONTROL SYSTEM Michael Slavin and Ralph W. Carp, Baltimore, Md., as-

signors to The Bendix Corporation, a corporation of Delaware Filed Mar. 13, 1968, Ser. No. 712,672 Int. Cl. B60t 8/04 U.S. Cl. 303--21 19 Claims ABSTRACT OF THE DISCLOSURE An anti-skid braking system for automobiles or trucks utilizing a control Channel for each wheel to be controlled, wherein the rotational velocity of the wheel is sensed and electronically differentiated to generate a D.C. voltage level correlative to wheel acceleration. This D C. voltage level is compared to a first reference level corresponding to a predetermined value of wheel deceleration which, if exceeded, causes the wheel speed at that instant to be memorized. If, during a predetermined time, the wheel speed has dropped by a predetermined percentage an error signal is generated which operates through a vacuum actuated hydraulic pressure modulator to release the braking force on the wheel so as to allow the wheel to accelerate. The vacuum actuated hydraulic pressure modulator is interposed in the hydraulic line between the master cylinder and the wheel cylinders of the wheel being controlled. A diaphragm and a cooperating displacement rod within the modulator body are positioned in accordance with the volumetric rate of air admitted to one side of the diaphragm by a solenoid valve which opens in response to the error signal. The displacement rod cooperates with a ball valve to normally allow free communication between the master cylinder and wheel cylinders; however, when air is admitted through the solenoid valve in response to the error signal the displacement rod is displaced so as to isolate the wheel cylinders from the master cylinder and additionally to rapidly attenuate the hydraulic pressure at the wheel cylinders. In response to the decreasing brake pressure, the wheel begins to accelerate towards vehicle speed. At a second reference level, corresponding to a predetermined acceleration, the error signal is extinguished thereby closing the solenoid valve. The modulator diaphragm and displacement rod then are repositioned so as to slowly increase the braking pressure, the rate of pressure increase being determined by the size of an orifice in the modulator diaphragm. If wheel acceleration, in spite of the increasing brake pressure, increases to a third reference level corresponding to a second predetermined acceleration rate, a modulator bypass valve is opened so as to shunt the aforementioned diaphragm orifice causing the displacement rod and diaphragm to be repositioned more rapidly thus increasing the brake pressure at a more rapid rate, which rate is determined by the size of the -bypass valve. This increase in brake pressure will eventually cause :the wheel acceleration to decrease below the third reference level at which time the bypass valve closes and brake pressure rate returns to a slow increase. If the wheel decelerates to the first reference level the cycle will be repeated. Additionally, electronic logic circuitry is provided to distinguish between the locked wheel and the stopped vehicle condition by continuing to release braking pressure on a wheel for a predetermined amount of time after the wheel drops below a threshold velocity. If the wheel remains at zero velocity during this time the stopped vehicle condition is indicated and brake pressure is re- Patented Feb. 10, 1970 TCC applied. If, however, wheel velocity should increase* during this time period, the locked wheel condition is 1ndicated and the anti-skid system is reactivated.

BACKGROUND OF 'TI-IE INVENTION The wheel braking pressure which can be exerted by a motor vehicle operator is sucient to cause the wheels to lock with resultant increase in stopping distance and reduced lateral vehicle stability. This is especially true when driving on low frictional coeicient surfaces. However, it is possible to optimize the braking characteristics of a wheeled vehicle under any tire-road interface condition by providing the vehicle with an anti-skid system which will modulate the braking pressure to a pressure which maximizes the frictional force at the tire-road interface.

Mu-slip curves, which are plots of the tire-road interface frictional force versus wheel slip, are well known in the art. These curves, which are empirically obtained, show a maximum mu in the range of 15 to 25% slip. Height and sharpness of this maximum point is generally dependent upon the nature of the tire-road interface and its condition. An anti-skid system which permits only that amount of brake pressure to be applied which causes a wheel slip corresponding to the maximum mu point on the mu-slip curve for the conditions then encountered provides optimum braking to the vehicle being controlled.

SUMMARY OF INVENTION It is one object of this invention to provide such an anti-skid system. Accordingly, an anti-skid braking systern has been devised for use on wheeled vehicles which will cause a braked vehicle to come to a controlled stop by modulating the hydraulic brake fluid pressure to maintain a wheel slip which will maximize within practical limits the force which is developed between the tire and road surface under conditions then encountered.

Another object of this invention is to provide an antiskid braking system which is compatible with conventional hydraulic braking systems and can be installed in the vehicle as an adjunct thereto, performing its operational functions therethrough. The anti-skid braking system which is the subject of this invention utilizes one essentially independent channel for each wheel or group of wheels which it is desired to control, the channels being connected into the hydraulic fluid lines of a conventional braking sy-stem and the vehicles electrical power supply, thus providing compatibility with conventional braking systems.

Briefly, this new anti-skid system operates as follows. An electrical counter mounted on or in close proximity to a wheel to be controlled senses the rotational velocity of that wheel so as to provide the basic input in the form of an electrical frequency proportional to wheel velocity to a control channel associated with that ywheel which generates a pulse train having a repetition rate related to the input frequency. A counter converts the pulses into a D.C. voltage proportional to wheel velocity. This velocity signal is differentiated to obtain an acceleration signal which is compared'with a predetermined reference voltage corresponding to a given deceleration (-gl). If and at the time the reference deceleration voltage is exceeded, the control channel memorizes the then attained velocity of the controlled wheel. If thereafter during a fixed time period the wheel speed decreases by a predetermined percent of its value at the time of memorization, a vacuum actuated pressure modulated air cylinder is activated to reduce the brake pressure at a predetermined rate. -Under the inuence of the reduced brake pressure the wheel will begin to accelerate towards vehicle speed at a rate determined by the frictional force at the tireroad interface. At a predetermined acceleration (-i-gl), the modulator is deactivated. The brake pressure thus begins to increase at a slow rate determined by modulator constructional details to be explained later. If wheel acceleration increases to a second predetermined acceleration, (+g2) a modulator bypass valve opens which causes the brake pressure to increase at a more rapid rate. When wheel acceleration falls below the +g2 reference level, the bypass valve closes and brake pressure increasereturns to the slow rate. Further increases in brake pressure eventually causes the wheel to decelerate the g1 reference level so that the cycle repeats.

The etect of this cycle is to produce a condition such that the wheel slip will tend to hang on the front side and just below the peak of the mu-slip curve during the slow brake pressure build up. Memorization of the wheel speed at the g1 point, coupled with the requirement for a Wheel speed decrease over a xed period of time before brake pressure is reduced ensures that pressure reduction takes place at a point beyond the peak of the mu-slip curve and, additionally, serves to prevent deceleration peaks caused by road shocks from triggering the anti-skid system. The rapid pressure drop triggered when the wheel attains the I-gz acceleration level prevents locking of the wheel, while the acceleration of the wheel towards car speed decreases wheel slip so that the wheel slip point being traced on the mu-slip curve passes back over the peak of the curve before the Ibrake pressure will once again slowly increase.

It is still another object of this invention to provide logic circuitry which can distinguish between a locked Wheel and a stopped vehicle condition and will automatically take appropriate action in accordance with this information.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a functional block diagram of an anti-skid system installed in a passenger motor vehicle having sensors on each Wheel and pressure modulators controlling each front wheel individually and the rear axle.

FIG. 2 is a typical mu-slip curve.

FIG. 3 is a plot of minimum slip values versus vehicle speed at memorization time.

FIG. 4 is a cut-away view of a pressure modulator made in accordance with the teachings of this invention.

FIG. 5 is a plot of various vehicle parameters versus time during a braked stop controlled by an anti-skid system of this invention. FIGS. 6A and 6B comprise a schematic of a single anti-skid control channel.

FIG. 7 is a schematic of a circuit for combining control channels to control a proup of Wheels in accordance in information received from the fastest wheel.

FIG. -8 is a schematic of a circuit for combining control channels to control a group of -Wheels in accordance with information received from the slowest wheel.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the description of "FIG, 1 it should be understood that control channel 44 is identical to control channel 43 and the description of the structure and operation of channel 43 is applicable to the structure and operation of channel 44, the only exception being that channel 43 controls the left front Wheel of the vehicle shown while channel 44 controls the right front wheel.

Referring to FIG. l, a brake pedal 10 may be depressed to pressurize the hydraulic uid in master cylinder 11 which thereby supplies a uid pressure in hydraulic lines 11a, 11b and 11e` in the conventional manner. Each hydraulic line has interposed in its path a vacuum actuated pressure modulator 13, 14 or 15 which,

when the anti-skid device is inoperative, allows hydraulic pressure to pass therethrough unimpeded, as will be later explained, t0 wheel brake cylinders 17 to 24. Wheel speed sensors 30 to 33, one mounted on and sensing the rotational velocity of each wheel, generate pulses linearly related to wheel velocity, which pulses are applied over electrical lines 30a, 31a, 32a and 33a to control channels 42, 41, 44 and 43, respectively. The pulses are converted to a D.C. voltage level in counters 45a, 45b and 45C. This D.C. voltage level which is proportional to wheel velocity can pass through threshold 51a, 51b or 51e only if the velocity of the sensed wheel is above a set threshold. In the case of channel 43 (and hence channel 44) the velocity signal is analyzed by logic circuitry 46c to recognize the locked wheel or the stopped vehicle condition in a manner to be described later. This logic function is performed in channels 41 and 42 by logic circuitry 46a after a velocity signal has been selected by select circuitry 50 which, it will be shown, can be designed to select either the higher or lower velocity signal.

It should be understood that the selection of a wheel having a lower velocity, while providing the shortest stopping distance, could result in some lateral stopping instability if the wheel having the lower velocity locks while the controlling wheel continues to turn due to higher mu forces on the controlling wheel. If the wheel having the lower velocity is selected to control there will be some sacrifice in stopping distance but with improved lateral stability. The optimum anti-skid system, it should now be obvious, is a system in which each wheel is controlled by an independent control channel, however, weight, space and economic considerations may require a compromise so that various types of select circuitry will be shown to illustrate the versatility of the invention in this respect.

Any change in the D.C. level, indicating a change in wheel velocity, is processed by differentiators 47a and 47e to produce a signal correlative to wheel acceleration, which signal in channel 43 is compared with predetermined reference voltage levels in comparators 58 to 60 while the acceleration signal developed in the hybrid channel 41 and 42 is applied to comparators 55, 56 and 57. It can thus be seen that channels 41 and 42 are combined by gate' 50 into a single channel which further processes only the selected velocity signal. Channels 43 and 44, however, are independent from one another and are independent from the combined channels 41 and 42. Additionally, except for the combining of channels 41 and -42 by gate 50, the channels are essentially identical to one another, thus the following discussion, for simplicity, will be restricted to a description of channel 43 with the understanding that the description applies equally to the other channels of the system.

As in the case of channels 41 and 42, the output of diiferentiator 47C is a voltage signal correlative to wheel acceleration (and deceleration) of the wheel controlled by channel 43. This acceleration signal is applied to comparators 58, 59 and 60. When the vehicle is braked, the wheels immediately start to slow down thereby producing an output from differentiator 47C corresponding to the wheel deceleration. Comparator 58 has applied .thereto a reference voltage level corresponding to the predetermined value of wheel deceleration 511). If wheel deceleration should exceed this reference level an error signal is generated by comparator 58 which.triggers one shot 65, suitably a monostable multivibrator. The leading edge of the one shotoutput pulse enables memory means 71, which suitably includes a low loss capacitor, to memorize a voltage corresponding to a predetermined percentage of present wheel speed, if, at any time during the period of the one shot output pulse the wheel velocity signal decreases below the memorized voltage, memory means 71 generates a latching signal which is applied to latching gate 95. Gate also receives an inhibit signal from comparator 59 when the acceleration exceeds a -l-gl acceleration reference level so that in the presence of a comparator 58 output, which indlcates that wheel deceleration exceeds the g1 reference level, the lack of an output from comparator 59, which indicates that wheel acceleration has not exceeded the -l-gl reference level and a Signal over line 79 which indicates that the brakes have been applied, gate 95 is latched open and a signal passes therethrough to amplifier 96 and vacuum modulator 15. The effect of this signal on modulator will be explained later.

Vacuum actuated pressure modulators 13, 14 and 15 are identical except for the fact that modulator 13 controls the brake pressure supplied to both rear wheels, whereas modulators 14 and 15 control the brake pressure supplied to the right front and left front wheels respectively. Refer now to the pressure modulator shown 1n greater detail in FIG. 4. A rubber diaphragm 310 d1v1des modulator body 301 into two chambers, 301a and 301b. Modulator chamber 301a communicates through check valve 302 and vacuum conduit 303 with a vacuum reservoir 304. A solenoid valve 314 having a winding connected to receive the output of gate 95 via amplifier 96 (FIG. l) operates when energized in response thereto to admit atmospheric air into modulator chamber 30111. Rubber diaphragm 310 is generally air-tight except for orifice 311 which allows restricted pneumatic communication between chambers 301g and 301b. Thus, with valve 314 deenergized, no atmospheric air can enter the modulator so that the vacuum on each side of diaphragm 310 is balanced and spring 316 biases diaphragm 310 to the right. When the solenoid valve opens in response to a gate 95 output, air is admitted into chamber 301b and thence through orifice 311 into chamber 301a and finally into vacuum reservoir 304. However, orice 311 is very much smaller than the effective air passage through solenoid valve 314 `so that air pressure builds up in chamber 301b thus forcing diaphragm 310 to the left against its biasing spring. A displacement rod 312 is connected at one end to the diaphragm and terminates at its opposite end in axially located pin 320 which, extending through valve seat 321, abuts valve ball 322. When diaphragm 310 is all the way to the right, corresponding to gate 95 being closed, valve ball 322 is liftedfrom valve seat 321 allowing free communication of hydraulic brake fluid from the master cylinder through valve ports 330 and 331 to the wheel cylinders. As the diaphragm moves slightly to the left carrying with it displacement rod 312, as would occur when gate 95 (FIG. l) opens initially, abutting pin 320 also moves to the left, allowing spring 323 to force ball 322 against seat 321, thereby isolating the master cylinder from the wheel cylinders and trapping a pressurized fluid in the wheel cylinder. At the instant the wheel cylinders become isolated the brake fluid pressure therein will be equal to the brake fiuid pressure developed in the master cylinder which, in the case of a panic stop, could be well above that pressure required to lock the wheels. Since, however, some finite time is required for the wheels to lock, at this instant the wheels have not yet attained the locked condition and additionally, diaphragm 310 continues to move to the left. This diaphragm movement carries displacement rod 312 and V packing gasket 340 also to the left. Expansible valve interior 341, whose left edge is defined by gasket 340, increases in volume causing the hydraulic fluid pressure trapped in the wheel cylinders to be thus attenuated. At maximum displacement of the diaphragm the volume of chamber 341 has increased to the point where the fluid pressure in the wheel cylinders has been substantially relieved and the braking on that wheel correspondingly released. This wheel is now accelerating.

Referring also to FIG. 1, as the wheel is now accelerating, the output of differentiator 47C changes from a deceleration to an acceleration signal so that comparator 58 no longer produces an output. It will, however, be

remembered that gate is latched open until the differentiator 47C output exceeds the -l-g1 reference level, which levent causes comparator 59 to generate a gate 95 inhibiting signal. Gate 95 will thus become unlatched and will close, deenergizing solenoid valve 314 thereby isolating the interior of the modulator body from the atmosphere. The atmospheric air entrapped in chamber 30111 continues to leak through orifice 311. However, since solenoid 314 is closed, the air in chamber 301b is not being replenished so that the diaphragm will now move slowly back towards the right carrying with it displacement rod 312. The rate at which the diaphragm moves towards the right is determined by the leakage rate between chambers 30111 and 301a. This is set by the size of the orifice which is quite small. The hydraulic fluid which had previously been displaced from the wheel cylinders into chamber 314 is slowly forced back into the wheel cylinders, thus slowly increasing the brake pressure on the wheel. It will be noted that the acceleration signal generated by diferentiator 47C is also applied to comparator 60, wherein it is compared to another acceleration level -l-g2 which is higher than the reference acceleration level -i-gl. If, in spite of the increasing brake pressure on the Wheel, the wheel should continue to accelerate so that reference acceleration level +g2 is exceeded, comparator 60 will generate an error signal which is amplified by amplifier and applied to a solenoid bypass valve 350 which, although normally closed, in response to this amplified error signal opens so as to shunt orifice 311. The air entrapped in chamber 301b can thus move more rapidly into chamber 301a, allowing diaphragm 310 and displacement rod 312 to move more rapidly to the right thus causing the brake pressure at the wheel cylinder to increase more rapidly. As the wheel cylinder brake pressure increases, the wheel accelerates more slowly so that at rst the --gz reference level is no longer exceeded. The error signal generated by comparator 60 is thus extinguished so that the bypass valve closes and diaphragm 310 resumes its slow movement to the right at the slow rate determined by orifice 311 so that brake pressure build up returns to the slow mode. As wheel acceleration decreases still further the -i-gl acceleration level is no longer exceeded so that the comparator 59 error signal is also extinguished thus extinguishing the inhibit signal on gate 95. This does not effect the slow build up of brake pressure but only allows gate 95 to be ready to receive the latching signal if the wheel decelerates below the g1 reference level and the cycle is repeated.

Referring now to FIG. 2, a typical mu-slip curve is seen. Mu is the frictional force developed at the tire-road interface and at automotive speeds is the major force tending to slow the vehicle. ln order to optimize the braking characteristics of a vehicle equipped with an anti-skid system, it is necessary that the mu developed at each wheel be maximized, that is, that the frictional force developed between the tire and the road surface be a maximum. Slip is the difference between vehicle velocity and wheel velocity where the wheel velocity is equal to its angular velocity times its radius. Slip is expressed mathematically as Tw if where r=radius of the wheel; w=rotational velocity of the wheel; and V=vehicle speed. It has been found empirically that the maximum point on a typical muslip curve for all investigated conditions of tire and road interface occurs between .15 and .30 slip. It has also been discovered through experimentation that under maximum mu conditions the highest stopping rate for a braked automobile that could be obtained was in the neighborhood of -lg. To insure that the anti-skid system will operate about the peak of the mu-slip curve the aforementioned g1 reference level has been set at a voltage corresponding to 1.8gs deceleration. Since, at the time of wheel speed memorization the Wheel must have obtained a 1.8g deceleration level while, as aforementioned, the maximum vehicle deceleration level is in the neighborhood of 1g, at the time of memorization there must be some wheel slip existing. Curve 120 of FIG. 3 shows the minimum value of slip with respect to the vehicle speed at the memorization time resulting from this difference in g level. It will be remembered that when the wheel attains a g1 level the wheel speed is memorized for a predetermined time during which the wheel speed is continuously monitored to determine whether it has dropped by a predetermined percentage. In an anti-skid system designed for a standard passenger vehicle in which the -gl reference level is set at 1.8g a decrease in car speed of 15% during a memorization period of S milliseconds has produced an anti-skid system having excellent braking capabilities.

Referring again to FIG. 3, curve 122 is a plot of minimum possible wheel slip versus vehicle speed if at the end of the 180 millisecond memorization period the wheel speed had decreased by exactly The minimum possible wheel slip at the time brake pressure is reduced will thus be a combination of the slips shown in curves 120 and 122, which is shown as curve 124. This curve shows that the minimum possible slip under the conditions stated is practically constant at all vehicle speeds and equal to approximately .15 slip. t

Referring now also to FIG. 5, where curve 125 is plot of vehicle speed versus time, curve 126 is a plot of wheel speed versus time, curve 127 is a plot of wheel acceleration versus time and curve 128 is a plot of brake pressure versus time. At a time prior to A the vehicle is unbraked so that the vehicle and wheel speeds are in synchronization. At time A the vehicle is braked so that the brake pressure rises rapidly and the wheel decelerates producing decreasing vehicle and wheel speeds. At time B the wheel decelerates past the g1 level and the wheel speed is memorized. At time C, which is less than 180 milliseconds after time B, wheel speed has decreased to 85 of the memorized speed so that brake pressure is now reduced. Due to inherent time lags in the hydraulic braking system the wheel will continue to decelerate for some finite time, attaining a maximum deceleration somewhere between time C and time D, the latter time being at the point of zero wheel acceleration. During this time, of course, the wheel would continue to slow down at a higher rate than the vehicle slow down rate so that wheel slip continues to increase. It can now lbe assured that the wheel has passed over the hump 115 of the mu-slip curve shown on FIG. 2 and is operating along the portion of the curve 117. Wheel speed. of course, hits a minimum point at time D and thereafter as the wheel now is accelerating wheel speed is increasing toward vehicle speed. At time E wheel acceleration becomes equal to the -l-gl acceleration level so that once again brake pressure is slowly increased. This s1ow increase in brake pressure plus the aforementioned inherent lag in the hydraulic braking system allows wheel acceleration to continue to increase until at time F the wheel has accelerated to the +g2 acceleration level. It will be remembered that at this time the pressure modulator bypass valve opens thereby allowing the brake pressure to increase rapidly as can be seen on curve 128 between time F and time G. During this time wheel acceleration has continued to increase past the +g2 acceleration level, nally reversing and returning below the -l-gg acceleration level at time G, thus closing the ybypass valve so that once again brake pressure is increased at a slow rate due solely to the orifice in the vacuum modulator diaphragm. At time H the wheel once more passes through the zero acceleration level and again begins to decelerate. During the time that the wheel has been accelerating, that is time D to time H, the slip as shown in FIG. 2 has been moving from the 117 portion ofthe mii-slip curve back over the hump 125 and into the 116 portion of the curve. At time H the slip is in close proximity to hump but on the portion of the curve 116. Since the wheel is now in the g region slip again will increase slowly under the influence of the slowly increasing brake pressure and once again approach hump 115. At a time just prior to I wheel slip reaches hump 115 on the mu-slip curve. A further increase in brake pressure unbalances the braking torque on the wheel for the existing road-tire interface conditions and the wheel proceeds rapidly toward lock-up as can be seen in the sharp rise in wheel deceleration at that time. At time I wheel deceleration exceeds the g1 level so that the cycle is once again repeated.

Refer now to FIGS. 6A and 6B which together comprise a typical channel for controlling a single wheel such as, in this embodiment, the left front wheel. In these figures, line terminations 1 to 8 of FIG. 6A are connected to line terminations 1 to 8 respectively of FIG. 6B. Sensor 33, which is driven by the left front wheel, for example, is suitably a variable reluctance alternating current type generator providing a signal having a frequency varying directly as the rotor speed and consequently as wheel speed, which signal frequency is impressed on counter 45C. The counter includes a limiter transistor which is normally biased into saturation, but which is biased non-conductive by negative half-cycles generated by the sensor. The limiter transistor output, therefore, will consist of pulses at approximately the regulated source voltage and will be of fixed amplitude with a frequency proportional to wheel speed. The counter also includes a counter circuit of the energy storage type to lwhich these speed pulses are applied so that a voltage proportional to actual wheel speed appears as the counter output on terminal 130.

Due to the back-to-back connection of diode 132 and the base-emitter diode of emitter-follower 140, the speed voltage on terminal is reproduced on the emitter of emitter follower with the exception that the emitter voltage cannot fall below a bias voltage supplied by the voltage divider comprised of resistors 135, 137, 138 and 141. The use of this bias voltage to eliminate velocity signals below a ser threshold will be explained later.

The emitter-follower emitter voltage, which, when above the threshold (bias) value, is a D.C. voltage proportional to wheel speed, is applied through the differentiator comprised of resistor and capacitor 147 to the base of transistor 150, which is part of a linear derivative amplifier comprising transistors 150, 151, 152, 153 and 154, whose gain is predetermined in the conventional manner to be a given voltage level per g of wheel acceleration. Temperature compensation is provided by diodes 155, 156 and 157. The amplified acceleration signal which appears on the emitter of transistor 154 is coupled to the base of transistor 160 which, together with transistor 162, comprise a comparator in the form of a Schmitt trigger, the triggering point of `which is set by the bias supplied to transistor 161 by the setting of potentiometer 163 which thus determines the bias condition of transistor 162. This triggering point is set at a voltage corresponding to the -gl reference level. In like manner the acceleration signal is also applied to the Schmitt trigger comprised of transistors 168 and 172, the biasing .of transistor as supplied by potentiometer 164 determining the triggering point which is set to a voltage corresponding to the -l-gl reference level. Also in like manner the acceleration signal is applied to the Schmitt trigger comprised of transistors 233 and 238 which triggers in accordance with the setting of potentiometer 230, which controls the bias on transistor 231, this last mentioned triggering point being the -l-g2 acceleration level. When the -gl deceleration level is exceeded the Schmitt trigger output appearing on the collector of transistor 162 triggers the one-shot comprised of transistors 181, 183 and 185, the output of which is applied through resistor 19t) to the base of transistor 193 so as to turn that transistor ofli. The one-shot output pulse also forward biases transistor 180, thus effectively preventing subsequent acceleration signals from reaching the one-shot. Transistor 193 is normally conductive so that there is normally impressed across capacitor 195 a voltage proportional to 85% of the instantaneous wheel speed. This last mentioned voltage is obtained via line 196 which applies to one side of capacitor 195 the voltage appearing on the emitter of emitter-follower 140 thus being the voltage proportional to wheel velocity, while the other side of capacitor 195 receives by a line 194 the emitter voltage of emitter-fol* lower 140 as divided down by the voltage divider comprised of resistors 137 and 138 this latter voltage being proportional to of the instantaneous wheel speed. When transistor 193 becomes non-conductive the voltage remaining across capacitor 195 is thus proportional to 85% of the instantaneous wheel speed. If during the memorization period, which is the period of the one-shot during which time transistor 193 is non-conductive, wheel speed drops by 15 since the voltage across capacitor 195 remains constant and the voltage on line 196 drops by 15%, the voltage on the gate of field effect transistor 200 passes through ground thus triggering it. Any output of the field effect transistor is amplified in the differential amplifier comprised of transistors 205 and 206 with this amplified output being applied to the base of transistor 210, which together with transistors 212 and 214 comprise an AND gate. Transistor 212 is maintained in the conductive condition whenever the -l-g1 acceleration level is not exceeded by the conductive state of the Schmitt trigger transistor 172. Transistor 214 is maintained conductive whenever the brake switch 220 is closed as would be the case when the brakes are applied by the vehicle operator. Normally, transistor 215 is shut olf due to the back-bias applied by diode 216 in its base circuit and diodes 217 and 218 in its emitter circuit. However, when transistors 210, 212 and 214 are conductive as is now the case, so that current is drawn through resistor 219, the voltage on the anode and hence also the cathode of diode 216 drops so that transistor 215 becomes conductive thus applying the gate output signal through the voltage divider comprised of resistors 223 and 224 to power amplifier 96, the amplified output of which energizes solenoid winding 226 of solenoid valve 314 which is part of the vacuum actuated pressure modulator of FIG. 4. Voltage in the collector of transistor 215 is fed back through resistor 221 to the base of transistor 210, hence latching this transistor in the conductive state so that gate 95 will remain open even though the wheel should accelerate below the g1 deceleration level as long as the brake pedal remains depressed and the -l-gl acceleration level is not exceeded. As soon as gate 95 opens transistor 212 collector voltage drops due to the aforementioned current fiow through resistor 219. This voltage drop is coupled to the base of transistor 230 which, although normally conductive, is now turned off thus causing its collector voltage to rise. This increased voltage is conveyed by line 207 to the base of the one-shot transistor 181 thus extinguishing the one-shot output pulse.

Since solenoid valve 314 lof FIG. 4 is now open the wheel will begin to accelerate toward vehicle speed in the manner previously described until the wheel has accelerated to the +g1 reference level, at which time the Schmitt trigger comprised of transistors 168 and 172 is triggered. During the time that the Wheel is accelerating above the -1-g1 acceleration level transistor 172 is turned off thus removing the forward bias on -gate transistor 212 so that the gate is disabled and unlatched.

While the wheel acceleration is below the -l-g2 reference level transistor 242 is saturated by the emitter voltage supplied through diode 240 and the base voltage produced by resistors 234, 236 and 237, and the emitter-base diode of transistor 231. Power is thus supplied through the voltage divider comprised of resistors 243 and 245 to the power amplifier 100 which thus energizes winding 246 of the normally open bypass solenoid valve 350 shown in FIG. 4, thus powering this solenoid shut. Whenever the wheel acceleration exceeds the reference -i-gg acceleration level Schmitt trigger transistor 233 is turned off so that its collector voltage rises, thus back-biasing transistor 242 and interrupting the power to solenoid winding 246 so that bypass valve 350 opens.

At low speeds anti-skid behavior may not be fully predictable and is not absolutely necessary for safe control of the vehicle. It would thus appear to be advantageous to provide a system which allows the braking system to operate normally when wheel speed is below a predetermined threshold value. However, the dropping of a wheel speed below threshold value may be due to one of two causes: either the vehicle itself and hence the wheel has dropped below the threshold speed; or, the wheel is operating on such a low friction coefficient surface that it has gone into lock-up before the control channel could function properly. It is thus imperative that the control channel be able to distinguish between the locked skidding wheel and the stopped vehicle conditions. The state of the control channel at the time the wheel speed drops below threshold speed is used to distinguish between these conditions. When the wheel is above the threshold speed, emitterfollower is conductive so that its collector voltage is depressed. Transistor 143 Whose base is connected to the emitter-follower collector is conductive, thereby charging capacitor 142 and supplying forward bias to transistor 158 through the voltage divider comprised of resistors 144 and 146. The current flow through transistor 158 and resistor 166 lowers the voltage on the anode of diode 169, thus back-biasing this diode. Schmitt trigger transistor 172 is biased in the conductive state as aforementioned by the voltage divider comprised of resistors 170, 171 and 232 so that a latchin-g signal is provided from the collector of this transistor to the base of gate transistor 212. When wheel speed drops below the threshold value the voltage on the collector of emitter-follower 140 is pulled toward source voltage, thus attempting to turn off transistor 143. However, the RC circuit comprised of capacitor 142 and resistors 139, 144 and 146 continues to hold transistor 158 in the conductive state for the time necessary to discharge the capacitor. In a practical, working system this time during which the control channel continues to operate after wheel speed has dropped below threshold speed has been set by the choice of resistor 139 and capacitor 142 to 0.4 second.

It now remains to be shown that the state of the control channel at the time wheel speed drops below threshold speed is determinative of whether the wheel is skidding or not. Of course, if the wheel speed always remains below threshold speed, the speed voltage on the emitter of emitter-follower 140 remains constant so that a zero acceleration signal is generated and the control channel remains inactive. If, however, at the time the wheel speed drops below threshold Value wheel deceleration had exceeded the g1 reference level and wheel velocity had dropped by 15 during the memorization period so that brake pressure is being released, the brake pressure will continue to be released during the 0.4 second delay period. If wheel skid is imminent, vehicle speed will be greatly in excess of wheel speed so that the wheel, in response to decreasing brake pressure, will increase in speed above the threshold speed during the delay period and the control channel will reacquire control. If wheel skid is not imminent, vehicle speed will `be close to wheel speed and the wheel will not accelerate over the threshold speed during the delay period. In the later case, at the end of the delay period, transistors 143 and 158 turn off and diode 169 becomes forward biased thus turning off transistor 172 and causing transistor 212 to become unlatched so that solenoid winding 226 is unenergized and braking control is returned to the vehicle operator. In a practical operating system, a threshold speed of five miles per hour has been selected.

Referring now to FIG. 7 a sensor 31, such as would be `connected to and sensing the velocity of the right rear wheel, drives counter 45a so as to generate a D.C. voltage correlative to wheel velocity which is applied to the base of transistor 250. Sensor 30, such as would be connected to and sensing the rotational velocity of the left rear wheel feeds counter 45b which thus generates a D.C. voltage correlative to the volocity of the left rear wheel, which voltage is applied to the base of transistor 251. The baseemitter diodes of transistors 250 and 251 perform the same function as diode 132 shown in FIG. 6A. The outputs of transistors 250 and 251 are applied respectively to the bases of differentially connected transistors 14051 and 140]: which are connected as emitter-followers. Only one of the differentially connected emitter-followers may be conductive at one time, the conductive emitter-follower being that emitter-follower to which the larger signal is applied. Hence that rear wheel having the higher velocity will control the output of the select circuitry which appears at the collectors of the emitter-followers (terminal Y) and the output which apepars at the common emitters of the emitter-followers (terminal X). Those circuit elements having letter sufiixed numeral designations correspond to similarly enumerated circuit elements in FIG. 6A, while terminals X and Y in FIG. 7 correspond to terminals X and Y in FIG. 6A. This method of circuit designation, which is also employed in FIG. 8, yet to be described, should clarify the position of the select circuit in the control channel circuitry.

Refer now to FIG. 8. As before, the output of counter 45a, which is a D.C. voltage proportional to the velocity of the right rear wheel, is applied to the base of transistor 252 While the output of counter 45h, which is a D.C. voltage proportional to the velocity of the left rear Wheel, is applied to the base of transistor 253 which is differentially connected with transistor 252 so that the transistor having the higher base voltage, corresponding to higher velocity, is turned off and the lower voltage corresponding to the velocity of the slower Wheel controls the circuit. The base emitter diode of transistor 252 or 253, Whichever is conductive, and the `base emitter diode of transistor 140C perform the same thresholding function as diode 132 and transistor 140 respectively of FIG. 6A.

From the aforementioned discussion it should be obvious that a vehicle could be equipped with an independent control channel such as channel 43 or 44 for each Wheel for any number of wheels. It should also be obvious that it is also possible to control a group of wheels by the use of select circuitry such as shown in FIG. 7 or 8. Other alterations and modifications will become apparent to one skilled in the art, therefore we do not wish to limit our invention to the specific form shown and accordingly hereby claim as our invention the subject matter including modifications and alterations thereof.

The invention claimed is:

1. In a wheeled vehicle having a wheel braking system whereby a wheel is braked by a braking force, an antiskid system having a source of electrical power and including a control channel comprising:

means for generating a first electrical signal proportional to the velocity of said wheel;

means for generating a second electrical signal pro portional to the acceleration and deceleration of said wheel;

means for generating a deceleration reference signal proportional to a predetermined deceleration reference level;

means for generating a first acceleration reference signal proportional to a first predetermined acceleration reference level;

means for generating a second acceleration reference signal proportional to a second predetermined acceleration reference level higher than said first acceleration reference level;

a first comparator means for comparing said second electrical signal with said deceleration reference signal to generate a memorization pulse having a predetermined time period;

memory means enabled by said memorization pulse for storing a predetermined percentage of the instantaneous value of said first electrical signal during said time period, said memory means being additionally for comparing said stored electrical signal with subsequent values of said first electrical signal during said time period to generate a latching signal when a predetermined relationship between said first electrical signal and said stored signal is attained;

a second comparator means for comparing said second electrical signal with said first acceleration reference signal to generate to a gating signal whenever said wheel acceleration is below said first acceleration reference level;

latching gate means responsive to said gating signal and said latching signal for generating a first error signal;

a third comparator for comparing said second acceleration reference signal with said second electrical signal for generating a second error signal whenever said wheel acceleration is above said second f acceleration reference level; and,

means responsive to said first error signal for attenuating said braking force, to `an absence of both first and second error signals for slowly restoring said previously attenuated braking force, and to said second error signal for rapidly restoring said attenuated braking force.

2. An anti-skid system as recited in claim 1 with additionally:

means responsive to said braking force for generating a latching gate qualification signal; and wherein said latching gate additionally includes:

qualification means responsive to said qualification signal for qualifying said latching gate.

3. An anti-skid system as recited in claim 1 with additionally means responsive to said memorization pulse for extinguishing said second electrical signal during said memorization pulse time period.

4. An anti-skid system wherein a plurality of control channels as recited in claim 1 are utilized, each said control channel responding to and for controlling a different wheel of said vehicle.

5. An anti-skid system as recited in claim 1 wherein said latching gate generates a memory disable signal simultaneously with said first error signal and with additionally:

means responsive to said memory disable signal for terminating said memorization pulse.

6. An anti-skid system as recited in claim 1 wherein said first electrical signal is referred to a first predetermined voltage level and said memory means includes:

a charge storage means having first and second ports, said first port being connected to receive said first electrical signal;

means for attenuating said first electrical signal in a predetermined manner;

normally conductive means for applying said attenuated first electrical signal to said second port when conductive and responsive to said memorization pulse for becomming non-conductive; and,

means having a triggering port connected to said second port for generating said latching signal when a second predetermined voltage level is impressed on said triggering port.

7. An anti-skid system as recited in claim 6 wherein said charge storage means is a capacitor and said normally conductive means comprises a transistor normally biased conductive and having its collector-emitter circuit connected to apply said attenuated first electrical signal to a second port, said transistor being biased nonconductive by said memorization pulse.

8. An anti-skid system as recited in claim 6 wherein said means for generating said latching signal comprises: a field effect transistor having gate, source and drain electrodes, said gate electrode comprising said triggering port; means biasing said field effect transistor for predetermining that said field efiect transistor generates an output signal when said second predetermined voltage level is impressed on said triggering port; and, amplifier`2 means responsive to said field effect transistor output" signal for generating said latching signal. y9. An anti-skid system as recited in claim 8 wherein said first predetermined voltage level comprises a common ground of said control channel.

10. An anti-skid system as recited in claim 8 wherein said first and second predetermined voltages are equal. 11. An -anti-skid system as recited in claim 1 wherein said means" for generating a first electrical signal comprises:

means for generating a train of pulses having a pulse repetition rate correlative to said wheel velocity;

pulsecoimting means responsive to said pulse train for generating a D.C. voltage level proportional to p said wheel velocity;

first means for shifting said D.C. voltage level a first predetermined amount; second means for shifting said shifted D C. voltage level a second predetermined amount, equal to but opposite in sense from said first predetermined amount, and including an output terminal upon which said doubly shifted D C. voltage appears; and,

means applying a bias voltage to said output terminal,

the total voltage on said output terminal comprising said first electrical signal and wherein said means for generating a second electrical signal comprises a difierentiator responsive to said first electrical signal, said bias voltage thereby constituting a threshold voltage.

12. An anti-skid system as recited in claim 11 wherein said first and second shifting means comprise two diodes connected back to back.

13. An anti-skid system as recited in claim 11 with additionally:

a discharge circuit;

means responsive to said first electrical signal for charging said discharge circuit when said first electrical signal is above said threshold voltage and for discharging said discharge circuit when said first electrical signal subsequently falls below said threshold voltage;

means responsive to the charge and discharging states of said discharge circuit for enabling said second comparator, said second comparator generated gating signal being extinguished when said second comparator is disabled.

14. An anti-skid system as recited in claim 13 wherein said means for enabling said second comparator comprises:

transistor means biased conductive when said discharge circuit is charged and discharging;

diode means shunting said second comparator but reverse biased when said transistor means is conductive.

15. An anti-skid system as recited in claim 1 including a low pressure sink and a high pressure fiuid source and wherein said braking force is hydraulically applied with hydraulic fiuid from a master cylinder to a wheel cylinder and said means for attenuating said braking force comprises a hydraulic fiuid pressure modulator interposed between said master cylinder and said wheel cylinder comprising:

a modulator body having a pneumatically sealed interior;

a leaky fiexible diaphragm for dividing said sealed interior into first and second expansible chambers; means for connecting said second chamber to said low pressure sink;

first valve means responsive to said first error signal for connecting said first chamber to said pressure fiuid source;

second valve means including a first port and a second port communicating with an expansible valve interior, responsive to the position of said diaphragm for allowing free communication between said first and second ports when said diaphragm is in a first position and for isolating said first port from said second port and expansible interior and for expanding said expansible interior when said diaphragm is urged out of said first position, the volume of said expansible interior being proportional to the displacement of said diaphragm from said first position;

means connecting said first port to said master cylinder;

means connecting said second port to said wheel cylinder;

spring means for biasing said diaphragm toward said first position; and,

third valve means responsive to said second error signal for shunting said diaphragm, said diaphragm being urged out of said first position by introduction of pressure fiuid into said first chamber when said first valve means is opened, and said diaphragm returning slowly to said first position when said first valve means is subsequently closed due to leakage of said pressure fiuid through said leaky diaphragm, and said diaphragm returning rapidly to said first position when said first valve means is closed and said third valve means is opened.

16. An anti-skid system as recited in claim 15 wherein said pressure fiuid is atmospheric air and said low pressure sink is a vacuum source.

17. An anti-skid system as recited in claim 16 wherein:

said first valve means comprises a bleed solenoid valve responsive to said first error signal connected to admit air into said first chamber when opened; and,

said third valve means comprises a shunt solenoid valve responsive to said second error signal and connected to allow pneumatic communication between said first and second chambers when open.

18. In a wheeled vehicle having a wheel braking system whereby at least a first and second wheel are braked by a single braking force, an anti-skid system including a hybrid control channel for dependently controlling said first and second wheel comprising:

means for generating a first electrical signal proportional to the velocity of said first wheel;

means for generating a second electrical signal proportional to the velocity of said second wheel;

means for selecting a controlling wheel by passing only one out of said first and second electrical signals in accordance with a predetermined characteristic of said first and second electrical signals;

means responsive to said passed electrical signal for generating a third electrical signal proportional to acceleration and deceleration of said controlling wheel;

means for generating a deceleration reference signal correlative to a predetermined deceleration reference level;

means for generating a first acceleration reference signal correlative to a first predetermined acceleration reference level;

means for generating a second acceleration reference signal correlative to a second predetermined acceleration reference level higher than said first acceleration reference level;

a first comparator means for comparing said third electrical signal with said deceleration reference signal t0 generate a memorization signal having a predetermined time period;

memory means enabled :by said memorization signal for storing a predetermined percentage of the instantaneous value of said passed electrical signal and for comparing said stored electrical signal with subsequent values of said passed electrical signal during said time period to generate a latching signal when a predetermined relationship between said passed electrical signal and said stored signal is attained;

a second comparator means for comparing said third electrical signal with said rst acceleration reference signal to generate a gating signal whenever the acceleration of said controlling wheel is below said first acceleration reference level;

latching gate means responsive to said gating signal and said latching signal for generating a first error signal;

a third comparator means for comparing said second acceleration reference signal with said third electrical signal for generating a second error signal whenever the acceleration of said controlling wheel is above said second acceleration reference level; and,

means responsive to said tirst error signal for attenuatng said single braking force, to an absence of both first 2 and second error signals for slowly restoring said single braking force, and to said second error signal for rapidly resto-ring said single 'braking force.

19. An anti-skid system hybrid control channel as recited in claim 18 wherein said first and second electrical signals are D.C. voltages and said selecting means selects one of said irst and second electrical signals in accordance With their D.C. voltage levels, said selecting means comprising:

first means for shifting said selected D.C. voltage a first predetermined amount; second means for shifting said selected and shifted D.C. voltage a second predetermined amount equal to but in opposite sense from said first predetermined amount, and including an output terminal upon which said selected and doubly shifted D.C. voltage appears; and, means applying a bias voltage to said output terminal, the total voltage appearing on said output terminal comprising said selecting means passed electrical signal,

References Cited UNITED STATES PATENTS 3,260,556 7/1'966 Packer 30S-21 3,260,555 7/1966 Packer 303-21 3,362,757 l/l968 Marcheron 303-21 3,398,995 8/1968 Martin 303-21 DUANE A. REGER, Primary Examiner U.S. Cl. X.R. 30S-20 

